The present invention generally relates to the testing of broadcasting systems, and more particularly to the testing of optical fiber paths containing optical splitters to divide broadcast signals over multiple secondary fiber paths.
Broadcasting systems such as cable television (CATV) networks utilize optical transmitters and optical fiber paths to broadcast signals from the head end to local customer areas. These optical signals may later be converted (at a conversion location, for example) to electrical signals which are carried over traditional coaxial cable to the final viewers or individual local customer areas. In typical CATV networks, a CATV laser transmitter at the head end sends a laser broadcasted CATV signal through an optical fiber path to distribute the CATV programming to local customer areas. As with most communications providers, CATV providers are concerned with service quality as perceived by the customer. Service quality is negatively impacted when the cable transmission system is improperly functioning. In addition, down time within an improperly functioning cable system infrastructure also translates to lost or potentially lost revenue and maintenance time for locating the damaged fiber.
In typical CATV networks, such as the one shown in FIG. 1, a CATV transmitter 110 at the head end sends a laser broadcasted CATV signal through an optical fiber path 120. The primary optical fiber path 120 often contains a splitter or passive optical network (PON) 130 which divides broadcast signals over multiple secondary fiber paths 140, which are distributed out to multiple local customer areas 145.
To test the integrity of the secondary fiber paths beyond the PON 130, Optical Time Domain Reflectometer technology (OTDR) is required. In the shown embodiment, an OTDR test signal is sent out along the primary optical fiber path 120, through the PON 130 and onto the secondary fiber paths 140. A returned OTDR test signal trace comprises the superposition of each secondary fiber trace. The analysis of such complex OTDR traces containing multiple fiber path data beyond the PON is not easily managed because the single trace does not distinguish between the different secondary fiber paths. Thus, if an event such as a break or bend along one of the secondary fibers occurs, it is difficult to determine which secondary fiber is experiencing the event using the single OTDR trace.
One method currently used for testing secondary fiber paths beyond a splitter is to temporarily cut each secondary fiber and determine which superposition peak is associated with each of the cuts. Another method is to use a polarization splitting device at the PON to direct polarization at different wavelengths beyond the splitter. Yet another method currently being used is analysis and simulation equipment which simulates what the OTDR trace should look like. The OTDR trace simulation may then be compared to the actual fiber trace to determine which secondary fiber path is experiencing a problem. These methods are both costly and inefficient ways of testing secondary fiber paths in a cable television system. In addition, these methods are imprecise in indicating which secondary fiber paths is experiencing a new event.
The present invention is a system and method for optically testing broadcasting system such as cable television (CATV) systems including an optical splitter or passive optical network (PON). The CATV system broadcasts a combined signal comprising an optical signal such as a CATV signal, a network communications signal such as a smart local access network (SLAN) signal, and a test signal such as an optical time domain reflectometer (OTDR) signal from a head end, along a primary fiber path. The invention uses wavelength-division multiplexing (WDM) to separate out the SLAN communications signal and the OTDR test signal at an optical router. The SLAN and OTDR test signals are sent to an optical switch co-located with the PON which is controlled by, for example a fiber administration system. The SLAN signal controls the optical switch. The CATV signal is sent directly to the PON to be distributed to the secondary fiber paths. Thus, the OTDR test signal is enabled to bypass the PON.
The optical switch has several outputs, each of which corresponds to a secondary fiber path. When testing is needed, the OTDR signal is transmitted along one of the secondary fiber paths, as directed by the optical switch. The OTDR test signal is reflected back to create a separate trace for each of the secondary fiber paths. When a problem occurs along one of the secondary fibers, such as a cut or bending of the fiber, the present invention enables determination of which fiber is experiencing the problem by allowing one to read the signal trace of each of the secondary fibers to find the trace containing a new event. Thus, each of the secondary fibers can be easily individually accessed.